Apparatus for the electrostatic separation of particulate mixtures

ABSTRACT

Apparatus for the electrostatic separation of a mixture of particles that exhibit difference in electrical conductivity, comprising: a conductive surface to which conducting particles lose their charge; feeding means for feeding the mixture of particles onto the conductive surface; an ionising electrode for ionising individual particles in the mixture of particles; and a first static electrode having the same polarity as the ionising electrode and which serves to generate a static electric field, the first static electrode being located sufficiently close to the ionising electrode that the static electric field acts continuously on the particles as they are ionised; wherein conducting particles are separated from non-conducting particles on the basis of their different retained charge after a period of contact with the conductive surface.

TECHNICAL FIELD

The present invention is concerned with a particle separator for theseparation of particulate mixtures comprising species that exhibitdifference in electrical conductivity and, more particularly, with theseparation of particulate mixtures comprising species that exhibitdifference in electrical conductivity through electrostatic separation.

BACKGROUND ART

Mineral separation plants used in the titanium mineral processingindustry world-wide consist essentially of similar process technologiesapplied in a manner that is often tailored to an individual ore bodiesseparation requirements. Dependent upon a wide number of factorsincluding particle size and shape, mineral grade, geology of the orebody, type of mineral species present and the physical characteristicsof said mineral species, a unique recovery process is applied tooptimise plant performance and satisfy operational and capital costtargets. Nevertheless, all titanium mineral processing plants in theworld utilise similar process technologies applied in varying ways toaccomplish their process needs.

Mining is carried out by firstly excavating the ore and subjecting it togravity concentration which isolates the heaviest particles into what istermed a heavy mineral concentrate. The heavy mineral concentrates aresent to a dry separation plant, where individual minerals species (ofwhich there may up to 20 or more present) are separated using theirdifferent magnetic, electrical or other physical properties, often atelevated temperatures. Separation equipment commonly includes but is notlimited to, high-tension electrostatic roll (HTR) and electrostaticplate (ESP) separators, as well as gravity and magnetic processes. Usingelectrostatic separation techniques the conductors such as rutile andilmenite are separated from the non-conductors such as zircon, quartzand monazite. These separators are extensively used for the separationof conductor and non-conductor mineral species typically found in thetitanium minerals industry.

A wide variety of electrostatic induced charge and ionised fieldseparators have been invented over the last 90 years however the devicesof existing commercial designs described below have undergone littlefundamental change in recent years.

Based on the charging mechanisms employed, three basic types of“electrostatic” separators include; (1) high tension roll ionised fieldseparators (HTR), (2) electrostatic plate and screen static fieldseparators (ESP and ESS herein called ESP) and (3) triboelectricseparators. ESP and HTR separators are the most commonly used today,although in recent times some interest has been directed towardstriboelectric separators. However their application remains limited tomineral species that can be contact charged and so they are suitable forseparations of non-conductor species only.

Customarily, HTR separators utilise a grounded roll that transports thefeed material through the high voltage ionising field (corona) whichcharges the particles by ion bombardment. Conducting particles losetheir charge to the earthed roll and are thrown from the roll bycentrifugal and gravity forces. Non-conducting particles are pinned tothe rotor and are transported further around the roll before theircharge either dissipates and they are thrown off or are removed byeither mechanical means (brush) or high voltage AC wiper.

ESP separators have an electrode designed to generate a static field andthe particles are charged by conductive induction. In their common formESP separators utilise a stationary grounded surface such as a plateover which the material flows, forming the connection to ground thatparticles must have to allow them to become charged by induction.Triboelectric separators do not use an electric field to effect particlecharging. Particle to particle and/or particle to surface chargingoccurs when particle species with different contact charging potentialare brought into contact with one another. The particle charge attainedcan then be utilised to effect a separation in a static electric field.

These three basic separation types are often not present alone in anymechanism, and the machine characterisation essentially refers to thepredominant or major separating effect. The present invention reliesprimarily on ion bombardment to charge the particles and so theoperation of a HTR separator is described in more detail below.

The main separating mechanism employed in HTR separators involves thefact that conductors will quickly release their charge to a groundedsurface and accordingly will be thrown off the rotating roll surface dueto the centrifugal or gravitational forces. Non-conductors being unableto conduct their charge to the grounded surface are pinned to the rollsurface. An “image force” pins the non-conductors to the roll and it canbe shown that the image charge developed on the conducting surface isrelated to the particle charge and its distance from the roll surface.If the particle charge is negative, it repels electrons in the imagevicinity in the conducting roll i.e. it generates its own positiveimage. This image has opposite polarity and the particle is attractedand pinned to the roll surface for this reason.

Thus the conductors tend to be thrown off the roll surface by theirnatural gravitational and centrifugal forces before falling through asplitter type collection means below and/or beyond the roll, dividingthe feed into a conductor rich fraction and a conductor poor, ornon-conductor, fraction.

Individual particle mass and shape partially determine the behaviour ofindividual particles in the separator and also the path followed by aparticular particle once it has left the roll surface.

The above description of the separation process describes a one-stageHTR separator. HTR separators typically incorporate up to 3 identicalstages with up to two starts or individual streams being treated in onemachine. Very simple separations such as removing highly conductiveilmenite from good non-conductors can often be effected with just onestage. Nevertheless, in a multi-stage machine each new stage follows thelast with material cascading from one stage to the next. Conductor ornon-conductor retreat configurations are common.

Each stage is similar to the first with feed chute, earthed roll,electrode and splitter system duplicated and arranged one above theother in a vertical configuration. Adjustment of splitters, electrodeposition and roll speed is typically done at each stage independently ofother stages.

In the treatment of mixtures of particles with a range of physicalcharacteristics including conductivity, particle size and density, it isnecessary to accurately set roll speed and relative positions of theelectrode and splitters to achieve effective separation. It is usuallynecessary to adjust not only the air gap between the roll and electrodebut also the alignment of the wire electrode and its backing memberrelative to the roll surface as well as the splitter positions,independently on each stage.

It is found in conventional HTR separators that not all particlescontact the roll for sufficient duration to enable the conductors to bedischarged and thrown Some of the particles which are fed onto the rollbounce up upon contact with the roll, as it rotates at relatively highspeed. This results in lower separation efficiency. In addition, feedstreams containing particles with low conductivity, such as leucoxene,may be incompletely separated as a result of incomplete discharge.Furthermore, feed streams in which there is wide particle sizevariation, particularly where the non-conductors are larger than theconductors, and feed streams containing fine particles below 75 micronsin size may be incompletely separated. The present invention provides ameans for minimising particle bounce and enhancing charge decay inconducting particles in order to improve separation efficiency.

DISCLOSURE OF THE INVENTION

Accordingly to one aspect of the present invention there is provided anapparatus for the electrostatic separation of a mixture of particlesthat exhibit difference in electrical conductivity, comprising:

-   -   a conductive surface to which conducting particles lose their        charge;    -   feeding means for feeding the mixture of particles onto the        conductive surface;    -   an ionising electrode for ionising individual particles in the        mixture of particles; and    -   a first static electrode having the same polarity as the        ionising electrode and which serves to generate a static        electric field, the first static electrode being located        sufficiently close to the ionising electrode that the static        electric field acts continuously on the particles as they are        ionised;    -   wherein conducting particles are separated from non-conducting        particles on the basis of their different retained charge after        a period of contact with the conductive surface.

It will be appreciated that the first static electrode ordinarily hasits leading edge closely adjacent the ionising electrode, and preferablyhas its leading edge located behind the ionising electrode with respectto the conductive surface. This ensures that the static electric fieldgenerated by the first static electrode acts continuously upon theionised particles both during and after the ionising process. This, inturn, ensures that there is a repelling action on all particles, bothconducting and non-conducting, tending to force them back onto theconductive surface. Accordingly, particle bounce is minimised andparticle contact with the conductive surface is maximised. As a result,the maximum opportunity for conducting particles to discharge theircharge to the conductive surface is provided, and therefore separationof conducting and non-conducting particles is enhanced. This effect ismost pronounced with the larger and heavier non-conductors since theseare most likely to bounce off the conductive surface and consequentlymisreport to the conductor stream. However, since these particles stillcarry most of the charge attained when ionised they are continuouslyrepelled by the first static electrode and therefore substantially lesslikely to join the conductor stream.

A convenient spacing for the ionising electrode and the first staticelectrode is in the range of 2 to 20 mm, preferably 5 to 10 mm, but thismay differ dependent upon the precise process conditions.

Advantageously, the ionising electrode is a corona electrode, and thecorona electrode includes a corona wire which is stretched in space.Advantageously, the corona wire is stretched to between two tensioningscrews that tension the wire above a backing bar, and this wire supportassembly may be attached by means of clips to the first staticelectrode.

It is desirable that the spacing of the ionising electrode and the firststatic electrode be adjustable, and therefore the apparatus may includeadjustment means for adjusting the spacing of the first static electrodeand the ionising electrode.

In one form of the invention, the corona wire position relative to thestatic electrode may be changed by adjusting the length of thetensioning screws that support the corona wire. An alternative andpreferred method of changing the relative ionising effect and the staticconductive induction effect is to use two or more high voltage powersupplies, one connected to the corona wire and at least one otherconnected to the first static electrode. In this way the ionising effectand hence the pinning forces and the conductive induction charge decayeffects can be decoupled, allowing the separation process to beoptimised. However, any other arrangement suitable for spacing theionising electrode and the first static electrode may be used. Forexample, the leading edge of the first static electrode may be spacedapart from the ionising electrode through a perforated plate, which actsas a spacer. In this arrangement the leading edge of the first staticelectrode would generally be fixed to one portion of the plate and theionising electrode, typically a corona wire, may penetrate any one of aplurality of perforations. Each of the perforations is spaced apart by adifferent distance from the first static electrode, and therefore thedistance between the corona wire and the first static electrode may beadjusted.

It will also be appreciated that the first static electrode acts uponthe particles as they are ionised, but may be of sufficient length toalso act on ionised particles. Since the first static electrode has thesame polarity as the ionising electrode it will assist in the chargedecay of the conducting particles. For example, if both the ionisingelectrode and the first static electrode have positive polarity, allparticles become initially positively charged due to ion bombardment.The charged particles will then start to give up their charge to theconductive surface by their own natural decay, but the first staticelectrode will also force their charge reversal as it endeavours toinduce a negative charge in them. Accordingly, the charge decay isperformed more quickly than in prior art devices, allowing theconducting particles to be thrown or lifted from the roll at an earlierpoint whilst at the same time forcing charged non-conductors to remainpinned to the roll surface for a longer time. In some forms of theinvention the static electric field generated by the first staticelectrode even acts on the conductive surface at a point beyond whereseparation of conducting and non-conducting particles occurs to continueto hold the non-conducting particles back to the conductive surface. Inparticular, very large or heavy particles are maintained on theconductive surface in this fashion. This ensures that they remain incontact with the surface for sufficient time to join the non-conductorstream. There may even be a second static electrode present in theapparatus which serves to extend the distance over which the staticelectric field is applied to the conductive surface. This embodiment ofthe invention, in particular, minimises sensitivity to particle sizevariation compared to prior art separators, thereby contributing toimproved separator performance.

In a further aspect of the present invention there is provided apparatusfor the electrostatic separation of a mixture of particles that exhibitdifference in electrical conductivity, comprising:

-   -   a rotating roll whose exterior surface is conductive;    -   feeding means for feeding the mixture of particles onto the        exterior surface of the rotating roll;    -   an ionising electrode for ionising individual particles in the        mixture of particles; and    -   a first static electrode having the same polarity as the        ionising electrode; and    -   a second static electrode having the same polarity as the        ionising electrode but positioned further around the rotating        roll so as to extend the static electric field generated by the        first static electrode;    -   wherein conducting particles lose their charge to the exterior        surface of the rotating roll after a period of contact therewith        and so are thrown off, while non-conducting particles are        retained on the exterior surface of the rotating roll.

In a particularly preferred embodiment of the present invention, theapparatus is a roll-type ionised field separator in which the conductivesurface is the exterior surface of a rotating roll.

Accordingly in a still further aspect of the present invention there isprovided an apparatus for the electrostatic separation of a mixture ofparticles that exhibit difference in electrical conductivity,comprising:

-   -   a rotating roll whose exterior surface is conductive;    -   feeding means for feeding the mixture of particles onto the        exterior surface of the rotating roll;    -   an ionising electrode for ionising individual particles in the        mixture of particles; and    -   a first static electrode having the same polarity as the        ionising electrode and which serves to generate a static        electric field, the first static electrode being located        sufficiently close to the ionising electrode that the static        electric field acts continuously on the particles as they are        ionised;    -   wherein conducting particles lose their charge to the exterior        surface of the rotating roll after a period of contact therewith        and so are thrown off, while non-conducting particles are        retained on the exterior surface of the rotating roll.

It is preferred that the rotating roll should rotate relatively slowlysince this increases the time that the particles spend within theelectric field produced by the or each static electrode and thereforeenhances separation. A preferred roll speed is around 150 to 250 rpm.Separation may also be enhanced by increasing the electrical fieldstrength, and typically voltages in the range of 15 to 40 kV may beapplied to all electrodes in the apparatus. The voltage applied to theelectrodes may be the same or different.

Advantageously, one or both of the first and second static electrode isa dielectric electrode. Such electrodes may be constructed in the mannerdescribed in International Application No. PCT/AUOO/00223 (WO 00/56462),the disclosure of which is incorporated herein by reference. The use ofa dielectric semi-conductor or non-conductor electrode is preferred, buta metal electrode may also be used. It will be appreciated that thedielectric electrode may easily be arranged in very close proximity withthe ionising electrode, and the close proximity of the electrode to theroll surface allows higher field strengths to be obtained. Metal staticelectrodes may also allow charge transfer to conductor particles whichstrike the electrode, and therefore some misreporting may occur.Nevertheless, they form a part of the invention, although a lesspreferred embodiment.

It is desirable for either static electrode, but particularly the secondstatic electrode, to follow the conductive surface. Thus, for a rotatingroll it is desirable that the electrode or electrodes be curved, andhave substantially the same degree of curvature as the surface of theroll. It will also be appreciated that conductor particles which arethrown off the roll could strike a static electrode with an impervioussurface, unless precautions were taken to prevent this. However, thismay be minimised through use of a finger electrode in which a pluralityof spaced apart fingers constitute the electrode. The fingers may besubstantially parallel and mounted to a base member, typically atregular intervals. Advantageously insulated wire fingers are cantileversupported to the base member. These may be installed concentric with thesurface of the roll. However, it will be appreciated that fingerelectrodes may have other configurations which generate a substantiallyuniform static electric field at the surface of the roll. For example,fingers may extend laterally across the surface of the roll with eachfinger positioned a predetermined distance from the roll so as togenerate a substantially uniform static electric field at the surface ofthe roll. The fingers in this embodiment may be supported by a memberlocated to one side of the roll to which all of the fingers are joined.

The individual fingers are spaced apart at a distance that provides areasonably uniform electric field strength at the roll surface. In apreferred embodiment of the invention the spacing is typically 20 to 75mm, but may be more or less dependent upon the other operationalperimeters. It will also be appreciated that the fingers need not becurved, but the maximum advantage is gained when they are curved toreflect the surface of the roll. In either case it will be appreciatedthat particles thrown from the roll will ordinarily pass between thefingers, and even on those occasions where they strike the fingers theywill normally do so in a glancing fashion and not be substantiallydeflected. Therefore, the provision of a finger electrode minimisesmisreporting through deflection of the particles.

According to a still further aspect of the present invention there isprovided a finger electrode comprising a plurality of substantiallyparallel spaced apart fingers mounted at regular intervals to a basemember, and further comprising appropriate electrical connections.

The apparatus of the present invention also advantageously includes amineral wiping brush to remove non-conducting particles from theconductive surface. This is typically a fibre or brass bristle brushwhich is in continuous contact with the conductive surface. Analternating current (AC) electrode may be located adjacent the brushenabling the charged particles to be neutralised. The non-conductingparticles may either fall from the roll or be brushed from the roll onceneutralised.

However, in addition, it has been found that conventional electrostaticseparators have a propensity for the conductive surface to attract andbecome coated with non-conductive organic or inorganic film after hoursor days of operation. As good electrical contact between the particlesand the conductive roll surface is highly desirable, it is preferredthat the apparatus further comprise cleaning means for cleaning theconductive surface. Since the cleaning means is generally an abrasivebrush or pad, it is desirable that it contact the conductive surfaceonly intermittently. A typical cleaner is a rotating abrasive linishroll, but an abrasive wire brush or an abrasive cloth or any otherconventional cleaner for such surfaces may be used. It is desirable thatthe abrasive cleaner only be used for short periods, as continuouscontact may result in rapid erosion of the conductive surface.

In a particularly preferred embodiment of the invention, a rotatingabrasive linish roll which has a rounded face and a flattened face isused. This roll rotates continuously or intermittently adjacent theexterior surface of the roll so as to contact it only when the roundedface and the conductive surface are juxtaposed. When the flattened faceand the conductor surface are close together, the flattened face doesnot reach to the surface of the roll.

In a still further aspect of the present invention there is provided acleaning device for cleaning a conductive surface on which separationoccurs in an apparatus for separating particles on the basis ofdifference in their electrical conductivity, comprising a rotating,abrasive roll which has a rounded face and a flattened face and rotatescontinuously or intermittently adjacent the conductive surface so as tocontact it only when the rounded face and the conductive surface arejuxtaposed.

In a still further aspect of the present invention there is provided anapparatus for the electrostatic separation of a mixture of particlesthat exhibit difference in electrical conductivity, comprising:

-   -   a rotating roll whose exterior surface is conductive;    -   a feed slide or roll feeder which feeds the mixture of particles        onto the exterior surface of the rotating roll;    -   a corona electrode for ionising individual particles in the        mixture of particles;    -   a first static electrode having the same polarity as the        ionising electrode and which serves to generate a static        electric field, the first static electrode being located        sufficiently close to the ionising electrode that the static        electric field acts continuously on the particles as they are        ionised; and    -   a mineral wiping brush which brushes the exterior surface of the        rotating roll;    -   wherein conducting particles lose their charge to the exterior        surface of the rotating roll after a period of contact therewith        and so are thrown off, while non-conducting particles are        retained on the exterior surface of the rotating roll until        brushed off.

The apparatus may also include an AC wiper which neutralises the chargeon the non-conducting particles.

Separation roll diameter is not critical. Typically the diameter of theroll in the apparatus described above will be between 150 mm and 1000mm, preferably between 200 and 400 mm. However, there is a balance ofissues regarding roll size in that single stage performance is improvedwith larger roll diameters, but the increased machine size and costneeds to be weighed up against the benefits of installing a greaternumber of smaller diameter rolls. These rolls may typically be used in amulti-stage apparatus.

The present invention also allows for a multi-stage particle separatorcomprising apparatus as described above in operative association with afurther particle separator or separators, which is typically alsoapparatus as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is an elevation showing apparatus in accordance with a firstembodiment of the present invention;

FIG. 2 is an elevation showing apparatus in accordance with a secondembodiment of the present invention, which includes a second staticelectrode;

FIG. 3 is an elevation of apparatus in accordance with a thirdembodiment of the present invention, which employs a finger electrode;

FIG. 4 is an isometric view of a finger electrode;

FIG. 5 is an elevation of apparatus in accordance with a fourthembodiment of the present invention, which illustrates a cleaningarrangement utilising a linish roll with a flattened face;

FIG. 6 is an elevation of a fifth embodiment of the invention, whichemploys an extended static electrode;

FIG. 7 is an elevation showing detail of the corona wire support memberof FIG's. 1 to 6; and

FIG. 8 shows an alternative means of locating the corona wire.

MODES FOR CARRYING OUT THE INVENTION

The apparatus 10, 110, 210, 310, 410 and 510 shown in FIGS. 1, 2, 3, 5,6 and 8, respectively, is a particle separator used to separateparticulate mixtures comprising species that exhibit difference inelectrical conductivity. In particular, the apparatus serves to separateelectrically conducting species from non-conducting species on the basisof their differing capacities to retain charge in a roll-typeelectrostatic separator. The devices are substantially similar andtherefore the overall operation shall be described with reference toFIG. 1 only, and variations shall be described with reference to FIGS. 2to 8. In view of the similarity of the devices, reference numerals inFIGS. 2 to 8 will be the same as those in FIG. 1 for similar features,except that the features will be numbered from 110, 210, 310, 410 and510 in FIGS. 2, 3, 4, 5, 6 and 8, respectively.

Referring to FIG. 1, a mixture of particulate material 11 is containedwithin hopper 12 and fed via a feed metering plate through feedingmeans, in this case a simple chute 13, onto roll 14. The particulatematerial 11 may also be fed onto the roll 14 by other suitable meanssuch as a roll feeder system, with or without a variable speed drive.The path followed by the feed material and the configuration of thechute may be varied in order to suit the nature of the feed material andother operating parameters, as would be well understood by the personskilled in the art.

The roll 14 has an exterior surface 15 which is made of a conductivematerial, in this case, a chrome material. The roll 14 rotates at aspeed of around 150 to 250 rpm and carries with it the particulatemixture 11 as it rotates. In this instance the roll 14 rotates in theclockwise direction, but it may also rotate in the anti-clockwisedirection if desired. The apparatus includes appropriate drivemechanisms and control mechanisms, as would be well understood by theperson skilled in the art. The roll diameter is typically 200 mm to 400mm in the apparatus shown. The roll 14 is mounted for rotation upon axle21, as would be well understood by the person skilled in the art.

The apparatus 10 includes an ionising electrode. This is a coronaelectrode comprising a corona wire 19. The apparatus also includes afirst static electrode 16 spaced apart from the exterior surface 15 ofthe roll 14. Detail of the corona wire support member 18 is best seen inFIG. 7. The corona wire support member 18 in each instance comprises atensioning screw 31 which screws into an appropriate cavity in a backingbar 33, located within a well in clip 32. The backing bar 33 is aninsulated metal rod which becomes an extension of the first staticelectrode 16, ensuring a large continuous static field at the rollsurface adjacent to the corona wire 19. A high voltage power supply isconnected to the corona support assembly 18 via lead 34. It is to benoted that the clip 32 is made of an insulating material andincorporates a rubber “0” ring 35 that clamps to the surface of thefirst static electrode 16, taking up any variation in its thickness.Since the backing bar 33 is insulated along its length, the wiretensioning screws 31 and the corona wire 19 are the only exposed highvoltage parts. The position of the corona wire 19 may be adjustedrelative to the first static electrode 16 by adjusting the length of thetensioning screws 31. This results in a change in the relativity betweenthe corona and static field strengths. However, this is preferably donethrough provision of two or more separate high voltage power supplies,one connected to the corona wire 19 and at least one other connected tothe first static electrode 16. Alternatively, the corona wire 519 couldextend through a perforation in the perforated plate 518 attached to theleading edge 517 of the static electrode 516, as shown in FIG. 8. Inthis embodiment, the spacing of corona wire 519 and the leading edge 517of static electrode 516 is determined by selection of the perforation inthe perforated plate 518 through which the corona wire extends.

As illustrated in FIG. 1, the particulate mixture 11 is fed onto theexterior surface 15 of the roll 14 the particles in the mixture becomecharged under the influence of the high voltage ionising field emanatingfrom the corona wire 19. Since the static electrode 16 has the samepolarity as the ionising electrode, this ensures immediate repulsion ofthe ionised particles by the static electrode which pushes the particlesonto the exterior surface 15 of the roll 14. In so doing, particlebounce is greatly reduced as the repulsion force on ionised particleacts immediately and continuously, even during the process of ionisationof the mixture. Furthermore, the static electrode 16 begins to induce aneutral (or even negative) charge to the conducting particles in theparticulate mixture 11 immediately, and continues this whilst theparticles are under the influence of the static electric field generatedby the static electrode 16. An electric field is present over a widearc, extending essentially from the point of ionisation 22 to a point 23on the roll 14 where the static electric field has substantiallydiminished. This ensures repulsion of charged non-conductors occurs overa large area of the roll, and specifically the area of the roll wheremost conductors are dislodged from the exterior surface 15 of the roll14. This is represented in showing a stream 24 of conductors which arethrown off the roll 14 by a combination of centrifugal force and gravityin a direction generally tangential to the roll. The conductor stream 24is collected in a manner known per se. Meanwhile, a mid-conductor stream25 is retained upon the exterior surface 15 of the roll 14 for a time,before charge decay causes these particles to be dislodged, butnon-conducting particles are retained on the roll until dislodged as anon-conductor stream 26.

The static electrode 16 and corona wire 19 are connected to one or twohigh voltage power supplies of like polarity, and may be operated at thesame or different voltages. A preferred embodiment includes separatehigh voltage power supplies of like polarity connected to eachelectrode, allowing each to be separately adjusted and optimised. Thestatic electrode 16 can be a metal conducting electrode or an insulateddielectric type such as described in International Application No.PCT/AU00/00223 (WO 00/56462), the disclosure of which is incorporatedherein by reference.

The non-conductors, unlike conducting particles, do not give up theircharge to the grounded exterior surface 15 of the roll 14. Thus, an“image force” pins the non-conductors to the roll, and likewise withmid-conductors, although charge decay does occur slowly. Therefore,mid-conductors are held on the roll 14 until charge decay occurssufficiently for them to be thrown off. This is some time after chargedecay of the conductors has been completed and these have been thrownoff. As shown in FIG. 1, ordinarily sufficient decay has occurred forthe combined centrifugal and gravitational forces at point 23 on theroll 14 to throw the mid-conductor stream 25 from the roll 14. However,the non-conductors remain on the roll until removed therefrom byconventional means. In the present apparatus, AC electrodes 20, 27neutralise the charge on the non-conductors as they pass by. However,since charge neutralisation may not be complete, a brush 28 is providedwhich sweeps the non-conductor stream 26 from the roll 14. Both thenon-conductors and the mid-conductors are collected in a manner knownper se.

The apparatus also includes a roll cleaning device which consists of alinish roll 29 brought into contact or out of contact with the exteriorsurface 15 of the roll 14 through mechanism 30 in a manner which wouldbe well understood by the person skilled in the art. Control means suchas proximity switches and the like will generally be present. The linishroll 29 will be brought into contact with the exterior surface 15 onoccasion to clean the surface, but removed from contact with the surfacefor the majority of the time in order to avoid excessive abrasion of thesurface. When in contact with the exterior surface 15, the linish roll29 slowly rotates to clean the surface of the roll 14 as it moves pastthe linish roll. Cleaning the exterior surface 15 of the roll 14 in thisway ensures that the chrome surface has adequate conductivity for chargedecay to occur at a sufficiently rapid rate. A chrome surface is readilycleaned in this fashion, but other, conventional conducting surfaces maybe used on the roll 14. In addition, other cleaning mechanisms may beused, and these include abrasive rubbing devices and/or rolls or othermechanisms that are brought into contact with the roll 14 on aintermittent basis.

Referring now to FIG. 2, it will be seen that a second static electrode131 is introduced. Therefore the conductor stream splits into twostreams, stream 124A which passes between the second static electrode131 and the roll 114 and stream 124B which passes between the secondstatic electrode 131 and the first static electrode 116. The secondstatic electrode 131 serves to extend the static electric field furtheraround the roll 114; compare point 23 in FIG. 1 to point 132 in FIG. 2.Thus, there is a greater zone in which the repulsion and charge decayeffects described above with reference to FIG. 1 occur, and thereforethe ionised particles are subjected to these forces for a longerduration. This ensures that there is greater separation efficiency.

Referring now to FIG. 3, it will be seen that the second staticelectrode 231 differs from that shown in FIG. 2 in that it is curved incross-section and extends substantially around the diameter of the roll214. This extends the static field to a point 232 at approximately thelowest point of the roll 214. In this embodiment of the invention therepulsion effect which maintains non-conductors on the roll 214 is theprimary effect enhanced.

The electrode used in FIG. 3 is a finger electrode of the typeillustrated in FIG. 4 since, from FIG. 3, it will be appreciated thatboth the conductor stream 224 and the mid-conductor stream 225 pass thestatic electrode 231.

Reference to FIG. 4 shows that the static electrode 231 comprises 5parallel fingers 233, 234, 235, 236, 237 cantilever supported at thebase by base member 238. In the embodiment of the invention shown inFIG. 3 the finger electrode 231 is installed with the individual fingerspaced apart from the exterior surface 215 of the roll 214, andinstalled concentric with the exterior surface 215. These individualfingers are spaced apart at a distance that provides for reasonablyuniform electric field strength at the roll surface 215, typically 20 to75 mm. It will be appreciated that the conductor stream 224 and themid-conductor stream 225 may pass through the finger electrode 231 withfew, if any, of the particles coming into contact with the electrode,and therefore these streams will not be substantially scattered. Even ifthere is contact, it will generally be glancing contact and theparticles may still be collected. The finger electrode 231 is insulatedfrom the ground and is charged to a high voltage with the same polarityas that of the other electrodes. However, the voltage may be of the sameor different magnitude to those employed in the other electrodes.

As shown in FIG. 6, a finger electrode 441 may be used which extendssubstantially around the roll 414 and replaces entirely conventionalstatic electrode designated 16 in FIG. 1. In this embodiment of theinvention the finger electrode 441 functions in the same manner as thestatic electrode 16, as described above for the electrode 231.

Referring now to FIG. 5, it will be appreciated that the apparatus mayinclude a novel cleaning mechanism which comprises a linish roll 334which has a flattened face 336 and a rounded face 335 which rotatesaround axle 337. The linish roll 334 rotates in a clockwise directionand therefore, as shown, when the flattened face 336 is adjacent theexterior surface 315 of the roll 314 it does not contact the roll.However, when rounded face 335 comes into a position adjacent the roll314, the surface of the linish roll bears upon the exterior surface 315of the roll 314, and continues to do so as the linish roll rotates. Itis not until a full rotation from one end 338 to the other end 339 ofthe rounded face 335 is completed that the contact between the linishroll 334 and the exterior surface 335 is broken. The linish roll mayrotate continuously or intermittently. Therefore, when the linish rollis stopped it is stopped in the position shown in FIG. 5 so that it doesnot make contact with the exterior surface 315 of the roll 314. Aposition sensor 333 and control system may be used to determine when theflattened face 336 of the linish roll 334 is adjacent the roll 314 andto stop or start its rotation.

INDUSTRIAL APPLICABILITY

The particle separator of the present invention is useful in separatingparticles which differ in their electrical conductivity such as in themineral processing industry. In particular, the invention is useful intitanium mineral process plants. However, many applications exist inareas such as scrap recovery, iron ore or industrial mineralbeneficiation processes, whereby this invention can be used to greatlyenhance product recovery and grades of material.

1-63. (cancelled)
 64. Apparatus for the electrostatic separation of amixture of particles that exhibit difference in electrical conductivity,comprising: a conductive surface to which conducting particles losetheir charge; feeding means for feeding the mixture of particles ontothe conductive surface; an ionizing electrode for ionizing individualparticles in the mixture of particles; and a first static electrodehaving the same polarity as the ionizing electrode and which serves togenerate a static electric field, the first static electrode beinglocated sufficiently close to the ionizing electrode that the staticelectric field acts continuously on the particles as they are ionized;wherein conducting particles are separated from non-conducting particleson the basis of their different retained charge after a period ofcontact with the conductive surface.
 65. Apparatus as claimed in claim64 wherein the first static electrode has its leading edge closelyadjacent the ionizing electrode.
 66. Apparatus as claimed in claim 65wherein the leading edge of the first static electrode and the ionizingelectrode are spaced apart by 2 to 20 mm.
 67. Apparatus as claimed inclaim 64 wherein the ionizing electrode is a corona wire.
 68. Apparatusas claimed in claim 64 wherein the first static electrode is ofsufficient length to also act on ionized particles.
 69. Apparatus asclaimed in claim 68 wherein the static electric field generated by thefirst static electrode acts on the conducting surface at a point beyondwhere separation of conducting and non-conducting particles occurs. 70.Apparatus as claimed in claim 64, further comprising a second staticelectrode which serves to extend the distance over which the staticelectric field is applied to the conductive surface.
 71. Apparatus asclaimed in claim 70 wherein the second static electrode comprises aplurality of spaced apart fingers.
 72. Apparatus as claimed in claim 71wherein the spaced apart fingers are substantially parallel. 73.Apparatus as claimed in claim 72 wherein the spaced apart fingers aremounted to a base member at regular intervals.
 74. Apparatus as claimedin claim 71 wherein each of the spaced apart fingers is shaped in theimage of the conductive surface.
 75. Apparatus as claimed in claim 74wherein the conductive surface is generally cylindrical and thereforeeach of the spaced apart fingers is curved with substantially the samedegree of curvature.
 76. Apparatus as claimed in claim 64 wherein one orboth of the first and second static electrode is a dielectric electrode.77. Apparatus as claimed in claim 64 wherein the conductive surface is achrome surface.
 78. Apparatus as claimed in claim 64, further comprisingcleaning means for cleaning the conductive surface.
 79. Apparatus asclaimed in claim 78 wherein the cleaning means are appliedintermittently to said conductive surface.
 80. Apparatus as claimed inclaim 79 wherein the cleaning means comprises a highly abrasive cleaner.81. Apparatus as claimed in claim 80 wherein the cleaner is a rotatingabrasive linish roll.
 82. Apparatus as claimed in claim 81 wherein thelinish roll has a rounded face and a flattened face and rotatescontinuously or intermittently adjacent the conductive surface so as tocontact it only when the rounded face and the conductive surface arejuxtaposed.
 83. Apparatus as claimed in claim 81, further comprisingmeans for bringing the linish roll in to and out of engagement with theconductive surface.
 84. Apparatus as claimed in claim 64, furthercomprising a mineral wiping brush to remove non-conducting particlesfrom the conductive surface.
 85. Apparatus for the electrostaticseparation of a mixture of particles that exhibit difference inelectrical conductivity, comprising: a rotating roll whose exteriorsurface is conductive; feeding means for feeding the mixture ofparticles onto the exterior surface of the rotating roll; an ionizingelectrode for ionizing individual particles in the mixture of particles;and a first static electrode having the same polarity as the ionizingelectrode and which serves to generate a static electric field, thefirst static electrode being located sufficiently close to the ionizingelectrode that the static electric field acts continuously on theparticles as they are ionized; wherein conducting particles lose theircharge to the exterior surface of the rotating roll after a period ofcontact therewith and so are thrown off, while non-conducting particlesare retained on the exterior surface of the rotating roll.
 86. Apparatusas claimed in claim 85 wherein the first static electrode has itsleading edge closely adjacent the ionizing electrode.
 87. Apparatus asclaimed in claim 86 wherein the leading edge of the first staticelectrode and the ionizing electrode are spaced apart by 2 to 20 mm. 88.Apparatus as claimed in claim 85 wherein the ionizing electrode is acorona wire.
 89. Apparatus as claimed in claim 85 wherein the firststatic electrode is of sufficient length to also act on ionizedparticles.
 90. Apparatus as claimed in claim 89 wherein the staticelectric field generated by the first static electrode acts on theconducting surface at a point beyond where the conducting particles areflung off the exterior surface of the rotating roll.
 91. Apparatus asclaimed in claim 85 further comprising a second static electrode whichserves to extend the distance over which the static electric field isapplied to the exterior surface.
 92. Apparatus as claimed in claim 91wherein the second static electrode comprises a plurality of spacedapart fingers.
 93. Apparatus as claimed in claim 92 wherein the spacedapart fingers are substantially parallel.
 94. Apparatus as claimed inclaim 93 wherein the spaced apart fingers are mounted to a base memberat regular intervals.
 95. Apparatus as claimed in claim 92 each of thespaced apart fingers has a curved shape and extends around the exteriorsurface of the rotating roll.
 96. Apparatus as claimed in claim 85wherein one or both of the first and second static electrode is adielectric electrode.
 97. Apparatus as claimed in claim 85 wherein theexterior surface of the rotating roll is a chrome surface.
 98. Apparatusas claimed in claim 85, further comprising cleaning means for cleaningthe exterior surface of the rotating roll.
 99. Apparatus as claimed inclaim 98 wherein the cleaning means are applied intermittently to saidconductive surface.
 100. Apparatus as claimed in claim 99 wherein thecleaning means comprises a highly abrasive cleaner.
 101. Apparatus asclaimed in claim 100 wherein the cleaner is a rotating abrasive linishroll.
 102. Apparatus as claimed in claim 101 wherein the linish roll hasa rounded face and a flattened face and rotates continuously orintermittently adjacent the rotating roll so as to contact the exteriorsurface only when the rounded face and the exterior surface arejuxtaposed.
 103. Apparatus as claimed in claim 101, further comprisingmeans for bringing the linish roll in to and out of engagement with theconductive surface.
 104. Apparatus as claimed in claim 85, furthercomprising a mineral wiping brush to remove non-conducting particlesfrom the conductive surface.
 105. Apparatus for the electrostaticseparation of a mixture of particles that exhibit difference inelectrical conductivity, comprising: a rotating roll whose exteriorsurface is conductive; a feed slide or roll feeder which feeds themixture of particles onto the exterior surface of the rotating roll; acorona electrode for ionizing individual particles in the mixture ofparticles; a first static electrode having the same polarity as theionizing electrode and which serves to generate a static electric field,the first static electrode being located sufficiently close to theionizing electrode that the static electric field acts continuously onthe particles as they are ionized; and a mineral wiping brush whichbrushes the exterior surface of the rotating roll; wherein conductingparticles lose their charge to the exterior surface of the rotating rollafter a period of contact therewith and so are thrown off, whichnon-conducting particles are retained on the exterior surface of therotating roll until brushed off.
 106. Apparatus as claimed in claim 105,further comprising an AC wiper which neutralizes the charge on thenon-conducting particles.
 107. Apparatus as claimed in claim 105 whereinthe first static electrode has its leading edge closely adjacent theionizing electrode.
 108. Apparatus as claimed in claim 107 wherein theleading edge of the first static electrode and the ionizing electrodeare spaced apart by 2 to 20 mm.
 109. Apparatus as claimed in claim 105wherein the first static electrode is of sufficient length to also acton ionized particles.
 110. Apparatus as claimed in claim 109 wherein thestatic electric field generated by the first static electrode acts onthe conducting surface at a point beyond where conducting particles areflung off the exterior surface of the rotating roll.
 111. Apparatus asclaimed in claim 105, further comprising a second static electrode whichserves to extend the distance over which the static electric field isapplied to the conductive surface.
 112. Apparatus as claimed in claim111 wherein the second static electrode comprises a plurality of spacedapart fingers.
 113. Apparatus as claimed in claim 112 wherein the spacedapart fingers are substantially parallel.
 114. Apparatus as claimed inclaim 113 wherein the spaced apart fingers are mounted to a base memberat regular intervals.
 115. Apparatus as claimed in claim 114 whereineach of the spaced apart fingers has a curved shape and extends aroundthe exterior surface of the rotating roll.
 116. Apparatus as claimed inclaim 115 wherein each of the spaced apart fingers is curved withsubstantially the same degree of curvature as the exterior surface ofthe rotating roll.
 117. Apparatus as claimed in claim 111 wherein one orboth of the first and second static electrode is a dielectric electrode.118. Apparatus as claimed in claim 105 wherein the exterior surface ofthe rotating roll is a chrome surface.
 119. Apparatus as claimed inclaim 105 further comprising a rotating abrasive linish roll which maybe applied intermittently to the exterior surface of the rotating rollto clean same.
 120. Apparatus as claimed in claim 119 wherein the linishroll has a rounded face and a flattened face and rotates continuously orintermittently adjacent the rotating roll so as to contact it only whenthe exterior surface and rounded face are juxtaposed.
 121. Apparatus asclaimed in claim 119, further comprising a moveable mounting forbringing the linish roll in to and out of engagement with the conductivesurface.
 122. A cleaning device for cleaning a conductive surface onwhich separation occurs in an apparatus for separating particles on thebasis of difference in their electrical conductivity, comprising arotating, abrasive roll which has a rounded face and a flattened faceand rotates continuously or intermittently adjacent the conductivesurface so as to contact the conductive surface only when the roundedface and the conductive surface are juxtaposed.
 123. A cleaning deviceas claimed in claim 122 wherein the rotating, abrasive roll is a linishroll.
 124. Apparatus for the electrostatic separation of a mixture ofparticles that exhibit difference in electrical conductivity,comprising: a rotating roll whose exterior surface is conductive;feeding means for feeding the mixture of particles onto the exteriorsurface of the rotating roll; an ionizing electrode for ionizingindividual particles in the mixture of particles; and a first staticelectrode having the same polarity as the ionizing electrode; and asecond static electrode having the same polarity as the ionizingelectrode but positioned further around the rotating roll so as toextend the static electric field generated by the first staticelectrode; wherein conducting particles lose their charge to theexterior surface of the rotating roll after a period of contacttherewith and so are thrown off, while non-conducting particles areretained on the exterior surface of the rotating roll.
 125. A fingerelectrode comprising a plurality of substantially parallel spaced apartfingers mounted at regular intervals to a base member, and furthercomprising appropriate electrical connections.
 126. A multi-stageparticle separator for the separation of particulate mixtures comprisingspecies that exhibit difference in electrical conductivity, comprisingapparatus in accordance with claim 64 in operative association with afurther particle separator or separators.